Organic materials have recently shown promise as the active layer in organic based thin film transistors and organic field effect transistors [see reference 1]. Such devices have potential applications in smart cards, security tags and the switching element in flat panel displays. Organic materials are envisaged to have substantial cost advantages over their silicon analogues if they can be deposited from solution, as this enables a fast, large-area fabrication route.
The performance of the device is principally based upon the charge carrier mobility of the semiconducting material and the current on/off ratio, so the ideal semiconductor should have a low conductivity in the off state, combined with a high charge carrier mobility (>1×10−3 cm2 V−1 s−1). In addition, it is important that the semiconducting material is relatively stable to oxidation i.e. it has a high ionisation potential, as oxidation leads to reduced device performance.
Compounds known in prior art which have been shown to be effective p-type semiconductors for organic FETs are dithienothiophene (DTT) (1) and its fused dimer α,α′-bis(dithieno[3,2-b:2′,3′-d]thiophene (BDT) (2) having the structures shown below [see reference 2-4].

In particular BDT, which has been extensively studied, has been shown to be an effective p-type semiconductor for organic FETs with a very high charge carrier mobility between 1×10−3 and 5×10−2 cm2 V−1 s−1 and very high current on/off ratios (up to 108). BDT also has been found in the solid state to have a completely coplanar formation, and to be more planar than oligomers of thiophene.
However, BDT has a high melting point and is very insoluble, therefore, if used as the active layer in an organic thin film transistor, it cannot be readily solution processed. As a result, for applications like FETs, prior art materials like BDT are usually deposited as a thin film by vacuum deposition, which is an expensive processing technique that is unsuitable for the fabrication of large-area films. To improve the solubility of BDT, several substituted derivatives have so far been synthesized (3), [see reference 4] but these have still required vacuum processing when used in thin film transistors.
Thus, an aspect of the present invention is to provide new materials for use as semiconductors or charge transport materials, which are easy to synthesize, have high charge mobility and are easily processible to form thin and large-area films for use in semiconductor devices. Other aspects of the invention are immediately evident to those skilled in the art from the following description.
The inventors have found that these aims can be achieved by providing new aryl copolymers of dithienothiophene (DTT).
Polymers containing DTT have been previously synthesised. DTT can be polymerised electrochemically but an insoluble material containing many structural defects is produced [see references 5,6]. Similarly copolymers with dithienopyrrole have also been made (4) [see reference 7]. DTT has also been incorporated into vinylidene polymers either via Knoevenagel (5) [see reference 8] or Wittig reactions (6) [see reference 9]. The latter gave insoluble polymers, whereas the former produced polymers that were soluble but had only moderate photovoltaic or photoconductive behaviour.

DTT dimers and homo polymers of DTT and copolymers of DTT and thiophenes are reported in the international patent application WO 99/12989 [reference 10]. However, no characterisation or details of the synthetic route of the polymers are disclosed.
One drawback of the thiophene based materials of prior art is the energy of the highest occupied molecular orbital (HOMO), around 4.9 eV. This electronic orbital energy is susceptible to electron removal via oxidation which, although often increasing the mobility, also increases the bulk conductivity of the material, and in addition, introduces a time dependence of conductivity when exposed to the air. High bulk conductivity can lead to a high OFF current in a transistor, and hence a low ON/OFF ratio, which is undesirable.
The inventors of the present invention have found that copolymerisation of DTT with other aromatic species provides a method to tune the position of the HOMO and hence improve the ON/OFF ratio and decrease the susceptability to air oxidation. It was also found that the use of substituted aromatics as comonomers will also combat the intrinsic insolubility of the DTT unit to yield solution processable polymers, thus avoiding the high costs associated with vacuum processing. Variation of the aromatic comonomer may also be used to enhance the liquid crystal behaviour of the polymers. The introduction of order in the polymer films leads to higher carrier mobility and liquid crystallinity is therefore a useful property. Thiophene boronic acids do not couple effectively therefore use of non-thiophene comonomers also allows for efficient Suzuki polymerisations.
A further aspect of the invention relates to reactive mesogens having a central core comprising a DTT-arylene unit, said core being linked, optionally via a spacer group, to one or two polymerisable groups. The reactive mesogens can induce or enhance liquid crystal phases or are liquid crystalline themselves. They can be oriented in their mesophase and the polymerisable group(s) can be polymerised or crosslinked in situ to form polymer films with a high degree of order, thus yielding improved semiconductor materials with high stability and high charge carrier mobility.
A further aspect of the invention relates to liquid crystal polymers like liquid crystal main chain or side chain polymers, in particular liquid crystal side chain polymers obtained from the reactive mesogens according to the present invention, which are then further processed, e.g., from solution as thin layers for use in semiconductor devices.